Seminars of Roel Snieder

The seminars listed here can be given upon request. A biographical sketch  can be found at the bottom of this page.

Workshop: The Art of Science

The current model for training researchers is very much like the medieval system where an apprentice follows a master for years of training. This model gives graduate students valuable hands-on experience. What often lacks in this educational model is an explicit transfer of skills and information at a rate and moment in time that is effective for acquiring research skills in a timely manner. For this reason I started the course "The Art of Science" at the Colorado School of Mines. I teach this material in different forms; as a semester-long course, as a short course of a few afternoons, or as a single one-hour seminar. Depending on the length of the course and the wishes of the audience I choose from the following topics:

The book The Art of Being a Scientist is now available from Cambridge University Press.

Title: Hydraulic fracturing, a tale of two continents

Hydraulic fracturing has recently revolutionized oil and gas production. The public concerns about this technique are significant, to an extent that limits the use of hydraulic fracturing in many regions. In this presentation I discuss these concerns and show that the societal response to hydraulic fracturing in North America and Europe is very different. But one can argue that on both continents the democratic decision-making process is broken down, albeit for different reasons. The result of this breakdown is that we are not asking ourselves the tough questions that we ought to ask ourselves. This is a presentation is for the non-specialist.

Title:  Variations and healing of the seismic velocity
Scattering of waves leads to a complexity of waveforms that is often seen by seismologists as a nuisance. And indeed, the complicated wave paths of multiple scattered waves makes it difficult to use these waves for imaging. Yet, the long wave paths of multiple scattered waves makes these waves an ideal tool for measuring minute velocity changes. This has led to the development of coda wave interferometry as a tool for measuring minute velocity changes in the laboratory and with field data. Combined with the use of noise cross correlations, seismic interferometry, this method is even more useful because it follows for a quasi-continuous measurement of velocity changes. I will show examples of detecting velocity changes in the laboratory, the earth's near surface, and in engineered structures. Perhaps surprisingly, the seismic velocity is not constant at all, and varies with the seasons, temperature, precipitation, as the weather does. In addition, the seismic velocity usually drops as a result of deformation. Most of these changes likely occur in the near surface or the region of deformation, and a drawback of using strongly scattered waves is that it is difficult to localize the spatial area of the velocity change. I will present laboratory measurements that show that a certain spatial localization of the velocity change can be achieved. One of the intriguing observations is that after deformation the seismic velocity recovers logarithmically with time.  The reason for this particular time-dependence is the presence of healing mechanisms that operate at different time scales. Since this is feature of many physical systems, the logarithmic healing is a widespread behavior and is akin in its generality to the Gutenberg-Richter law.

Title: Focusing waves in unknown media

In many applications, such as imaging, one needs to focus wavefields. In general one needs to know the medium to focus waves, and limitations on the properties of the medium can hamper adequate focusing. In this presentation I show a method to focus waves in an unknown medium. The method is based on inverse scattering methods as originally developed in quantum mechanics. I show, using simple examples, that the so-called Marchenko equation provides the Green's functions that accounts for the wave propagation from the acquisition surface to any arbitrary point in the medium. These Green's functions account for the wave propagation in the unknown medium and can directly be used to focus or image waves.

Title: Facing the main challenges in Carbon Capture and Sequestration (download paper)

Capturing CO2 and injecting it in the subsurface is often seen as the main tool to prevent man-made global warming. The following questions must be answered before this process can be used on a scale that actually makes a difference in preventing climate change. (1) How can the cost of this process be reduced from its projected cost of 150 billion dollars per year? (2) How can this the capture and injection be up-scaled with a factor 1000 beyond current capabilities? (3) How can we predict and monitor leakage? Many action alternative to carbon capture and sequestration likely to be much cheaper and save energy as well.

Tutorial: seismic interferometry, who needs a seismic source? (download ppt)

Seismic interferometry is a technique for imaging without coherent sources. The idea is to combine waveforms, generated by ambient noise, that are recorded at different receivers in a way to provide the waves that would propagate between these receivers as if there was a source at one of these receivers. This obviates the need to have a soure located at one of the receivers. In the tutorial I cover different formulations of the theory that explain seismic interferometry, and present examples with field data that show the possibilities that are opened up with this new technique. With the advent of permanent networks of seismometers in exploration seismology and global seismology, seismic interferometry opens up new methods for imaging and monitoring.

Title: Ten ethical questions for scientists and engineers

Science and engineering in the broadest sense not only help us better understand the world in which we live; these fields also increase the power that we hold over the world. Unfortunately, neither science nor engineering comes with a recipe how to use that power. This idea is captured by the writer Goswani* who states that "Creativity unguided is a two-edged sword. It can be used to enhance the ego at the expense of civilization. One must apply creativity with wisdom." Helping students grow the wisdom how to use science and engineering responsibly is one of the goals of teaching ethics. In addition, students benefit from learning how to make ethical decisions in the daily practice of science. Ethics training is now mandated by the National Science Foundation for all students and postdocs that are supported by this organization. In response to this requirement, the Colorado School of Mines has developed the graduate course "Introduction to Research Ethics" (SYGN502).

* Goswani, A., The self-aware universe, Penguin Putnam Inc., New York, 1995.

Title: Diversity in the research environment

The modern research environment is increasingly diverse. This is partly due to increased globalization, but within the United States and Europe this is also due to a change in the demographics. This increased diversity poses challenges and opportunities for optimally collaborating and communicating. The increasing international diversity of research groups is, unfortunately, not accompanied by an increased participation of US minorities in research. I present some of the roadblocks that hamper an increased diversity in research that include lack of knowledge or appreciation of other cultures, deeply ingrained prejudices, fear, social inequality, and perhaps most importantly, an inability to see and appreciate our common humanity. Identifying these impediments may help remove or overcome them, so that we can take advantage of the cultural and intellectual enrichment of a diverse research environment.

Title: Extraction of the Green's function from ambient fluctuations for general linear systems

The extraction of the Green's function of acoustic and elastic waves from ambient fluctuations is by now a technique that is theoretical  well-described and that has succesfully been used in different applications. I show theoretically that the principle of the extraction of the Green's function can be generalized to a wide class of linear systems.  These new applications include the diffusion equation, Maxwell's equations, a vibrating beam, and the Schroedinger equation.  For systems that are invariant for time-reversal it suffices to have sources of ambient fluctuations on a surface that bounds the region of interest. When the invariance for time-reversal  is broken, as for example in the case of the diffusion equation or for wave propagation in attenuating media, one also needs sources of ambient fluctuations throught the  volume. This work opens up new opportunities to extract the Green's function from ambient fluctuations that include electromagnetic fields in conducting media, flow in porous media, wave propagation in attenuating media, monitoring of mechanical structures, and quantum mechanics.

Title: Extracting the building response from incoherent waves

Structures such as buildings or bridges are often instrumented with acellerometers to monitor the vibrations. Since the excitation of these structures usually is incoherent, these recordings do not directly give the impulse response (the response to an impulsive loading) of these structures. I show how seismic interferometry can be used to extract the impulse response from a building from incoherent vibrations recorded in a building after an earthquake. I also show that depending on the data-processing that is applied, either the propagating waves or the normal modes of the buliding can be retrieved. With this apprach the response of the building can be separated from the coupling of the building to the subsurface. In this seminar I show the theory and apply this to the motion recorded in the Millikan Libary in Pasadena (California).

Title: Coda Wave Interferometry

Multiple scattered waves are not very useful for deterministic imaging in complicated media because there is no known algorithm to construct such an image. Because multiple scattered waves have long wave-paths, these waves are very sensitive to small changes in the medium. Coda wave interferometry is a new technique that can be used to detect minute changes in a strongly scattering medium using changes in the multiple scattered waves over time. This technique is analogous to speckle pattern interferometry as used in optics, but takes advantage of the phase information in recorded waves. Because of its modest hardware requirements, coda wave interometry has a large number of applications. These include geotechnical applications (dam-monitoring, tunnel monitoring), the evaluation of hazards (volcano and fault monitoring), non-destructive testing, locating earthquakes, and monitoring of hydrocarbon reservoirs.

Title: Time-reversed imaging as a diagnostic of wave and particle chaos

Chaotic behaviour of particles concerns the stability properties of trajectories under perturbations of initial conditions. For waves, chaotic behaviour is less clearly defined. Both Newton's law and the Helmholtz equation are symmetric under time-reversal. This means that particles or waves emitted by a source at t=0 should refocus on the source when their propagation is reversed in time. Chaotic behaviour will prevent this to occur. This idea is tested for a system of very strong scatterers through which particles and wave propagate. Analytical expressions are derived for the critical perturbations of the initial conditions of both waves and particles. It is shown that the resulting behaviour of waves and particles are fundamentally different with critical length scales ranging over 15 orders of magnitude. The analytical results are illustrated and confirmed by numerical simulations.

Title: The arrow of time

It is great irony of science that the most fundamental concepts are often most dificult to understand. The concept "time" is an important example of this. The laws that describe the basic forces in nature are symmetric for time reversal. This means that they do not change when one changes the direction of time by replacing the time t by -t. However, this clearly contradicts our experience; we perceive a direction of time. This direction is called the "arrow of time." Different arrows of time can be distinguished: the thermodynamic arrow of time, the biological arrow of time, the radiative arrow of time, the mechanical arrow of time and the cosmological arrow of time. The relation with natural laws that are not invariant for time reversal is discussed and some pitfalls are shown in "deriving" equations with a direction of time from the fundamental laws of physics. The symmetry of time reversal has important applications in geophysics, examples are shown of this. The final question remains: "what explains the arrow of time that seems to pervade our daily experience?"

Title: Earthquake prediction, a political problem?

Summary: Earthquakes are among natural hazards that threaten society. For this  reason earthquake prediction is a field of research that arouses considerable nterest. An overview of the earthquake prediction problem is given. I show that the earthquake-prediction activities of scientists confront decision-makers with a fundamental trade-off between information and probability that the eartquake indeed occurs. However, this does not imply that scientists cannot contribute to alleviate the danger posed by earthquakes.

Biographical sketch

Roel Snieder holds the Keck Foundation Endowed Chair of Basic  Exploration Science at the Colorado School of Mines. He received in 1984 a Masters degree in Geophysical Fluid Dynamics from Princeton University, and in 1987 a Ph.D. in seismology from Utrecht University. In 1993 he was  appointed as professor of seismology at Utrecht University, where from 1997-2000 he was appointed as Dean of the Faculty of Earth Sciences. Roel served  on the editorial boards of Geophysical Journal International, Inverse Problems, Reviews of Geophysics, the Journal of the Acoustical Society of America, and the European Journal of Physics. In 2000 he was elected as Fellow of the American Geophysical Union. He is author of the textbooks  "A Guided Tour of Mathematical Methods for the Physical Sciences", "The Art of Being a Scientist", and "The Joy of Science" that are published by Cambridge University Press. Roel is a corresponding member of the Royal Netherlands Academy of Arts and Sciences. In 2011 he was elected as Honorary Member of the Society of Exploration Geophysicists, and in 2014 he received a research award from the Alexander von Humboldt Foundation. In 2016 Roel received the Beno Guterberg Medal from the European Geophysical Union and the Outstanding Educator Award from the Society of Exploration Geophysicists. From 2000-2014 he was a firefighter in Genesee Fire Rescue where he served for two years as Fire Chief.